In the global system for mobile communications (GSM) a transcoder rate adaptor unit (TRAU) is provided in the radio access network. The TRAU enables the use of different transmission rates over the Abis interface between a base station controller (BSC) and one or more base transceiver stations (BTSs). The switching sub-system to which the radio access network provides an interface for mobile stations operates at a bit rate of 64 kbps, which is the rate for which the mobile services switching centre (MSC) of the switching sub-system is designed.
It is well-known in GSM systems for adaptive multi-rate (AMR) to be supported, which enables the rate between the BSC and a BTS to be varied between one of eight different rates. Eight codecs are provided in the BSC, and an appropriate codec is chosen to ensure the required rate is used. For example, a reduced rate may be used where the quality of the air interface is known to be poor.
The codecs for selecting different rates are provided in the TRAU. The TRAU is logically associated with the BSC. In accordance with the particular codec selected for transmission on the Abis interface, a particular TRAU frame format is provided on the Abis interface. This frame is specific to the specific codec used. For example for AMR coding, the data rate in the TRAU frame is different for each codec, and the data bits are located in different positions for each TRAU frame. In the AMR example, where eight different codecs are provided, eight different TRAU frame formats are specified.
The third generation partnership project (3GPP) is proposing to introduce a flexible layer one (FLO) concept to support the handling of current and future services in the GERAN (GSM/EDGE radio access network). As part of current standardisation work in 3GPP, FLO is being introduced to the circuit switched part of the GERAN system.
The advent of flexible layer one in 3GPP systems provides the possibility for more flexibility in encoding techniques used in the radio access network. Potentially, therefore, in the future there may be provided many different types of codecs in the TRAU. Consequently, this will require many different types of TRAU frame formats. The advent of FLO may disadvantageously lead to the need to specify a number of different TRAU frame formats, by way of standardisation or otherwise.
It is an aim of the invention to address the above-stated problem.